wherein:

X represents bond or CH₂;

R₁ represents a keto group or (H,H);

R₂ represents OH or OMe;

R₃ represents H, OH or OMe;

R₄ and R₅ each independently represent H or OH;

R₆ represents —R₇, —C(O)R₇, —(CH₂)₂—O—[CR₂₁R₂₂—O]_a—C(O)—R₂₃; —CR₂₁R₂₂—O—C(O)—R₂₃; —POR₁₉R₂₀, —PO(OR₁₉)(OR₂₀) or Y—R₁₅;

R₇ represents —(CR₈R₉)_m(CR₁₀R₁₁)_pCR₁₂R₁₃R₁₄;

R₈ and R₉ each independently represent C1-C4 alkyl, C2-C4 alkenyl or C2-C4 alkynyl, any of which groups may optionally be substituted with —PO(OH)₂, —CF₂PO(OH)₂, —OH, —COOH or —NH₂; or R₈ and R₉ each independently represent H, trifluoromethyl or F;

R₁₀, R₁₁, R₁₂, R₁₃ and R₁₄ each independently represent C1-C4 alkyl, C2-C4 alkenyl or C2-C4 alkynyl, any of which groups may optionally be substituted with —PO(OH)₂, —CF₂PO(OH)₂, —OH, —COOH or —NH₂; or R₁₀, R₁₁, R₁₂, R₁₃ and R₁₄ may be independently selected from H, —(CR₈R₉)_qNH₂, —(CR₈R₉)_qOH, CF₃, F, COOH; or R₁₀ and R₁₁ or R₁₂ and R₁₃ or R₁₃ and R₁₄ may be taken together with the carbon to which they are joined to form a C3-C6 cycloalkyl or a 3- to 6-membered heteroalkyl ring that contains one or more heteroatoms selected from N, O and S and that is optionally, substituted with up to 5 —(CR₈R₉)_qOH, —(CR₈R₉)_qNH₂ or COOH groups;

Y=bond, —C(O)—O—; —(CH₂)₂—O—C(O)—O—;

R₁₅ represents

R₁₆ are each independently H or OH;

R₁₇ is independently selected from H, OH and NH₂;

R₁₈ is independently selected from H, —CH₃, —CH₂OH and —COOH;

provided however that no more than 2 groups selected from R₁₆, R₁₇ and R₁₈ represent H or CH₃;

R₁₉ and R₂₀ each independently represent H or C1-C4 alkyl or R₁₉ and R₂₀ together represent ═CH₂;

R₂₁ is independently selected from H, CH₃;

R₂₂ is independently selected from H, —CH₃, —CH═CH₂, —CH₂Cl, —CHCl₂, —CCl₃, —CH(OH)Me, —CH₂OH, —CH₂CH₃, —CH(Cl)Me;

R₂₃ is independently R₇, Y—R₁₅ or a 5- or 6-membered aryl or heteroaryl ring optionally substituted with between one and three groups selected from OH, F, Cl, Br, NO₂ and NH₂;

a represents 0 or 1;

m, p and q each independently represent an integer between 0-4;

provided however that the R₇ moiety does not contain more than 12 carbon atoms and does contain at least one functional group selected from —PO(OH)₂, —CF₂PO(OH)₂, —COOH, OH or NH₂;

or a pharmaceutically acceptable salt thereof.

a) R₇ contains 7 or fewer carbon atoms;

b) R₇ contains 5 or fewer carbon atoms;

c) R₇ contains two groups selected from —PO(OH)₂, —CF₂PO(OH)₂, —OH, —COOH and —NH₂; and

d) R₇ contains at least one functional group selected from —COOH, OH and NH₂.

wherein R₇ is selected from —CH₂OH, —C(CH₂)₂CH₂OH, —CH(OH)—CH₂OH, —CH(CH₂OH)—CH₂OH and —C(CH₂OH)₂—CH₃.

wherein R₁₆ and R₁₇ are OH and R₁₈ is CH₂OH; or R₁₆ is OH, R₁₇ is NH₂ and R₁₈ is CH₂OH; or R₁₆ and R₁₇ are OH and R₁₈ is COOH; or R₁₆ is OH, R₁₇ is NH₂ and R₁₈ is H; or R₁₆ is OH.

(a) reacting a compound of formula (II):

where R_A represents H or (CH₂)₂—OH

or a protected derivative thereof with a compound of formula (III):

HO—R₆ (III);

or an activated derivative thereof

wherein the group R₆ is as defined above for compounds of formula (I);

or a protected derivative thereof; or

(b) converting a compound of formula (I) or a salt thereof to another compound of formula (I) or another pharmaceutically acceptable salt thereof; or

(c) deprotecting a protected compound of formula (I).

wherein R₁₆ is OH.